Once you've passed the beginner phase of resistance training, it can become more difficult to gain muscle without making changes to your nutrition. Generally, muscle requires fuel to build, which is difficult, if not impossible to do if you're not eating in a calorie surplus. What does that mean? A calorie surplus is pretty simple - this means you're consuming more calories than you're burning throughout the day. When combined with a progressive resistance/weight training routine this can result in muscle growth and increased strength. Beyond the gains, spending some time in a calorie surplus can increase your energy, stabilize hormone levels and ultimately increase your metabolism as you put on more metabolically active tissue (muscle!)
Now, a calorie surplus with the intention of putting on muscle doesn't mean you should forget all of your healthy eating habits and eat like it's your first year of college. You want to continue making sure your body is getting the nutrients it needs to fuel your increasingly difficult workouts. It is true that you will likely gain some fat mass in addition to muscle while eating in a calorie surplus, but by continuing to eat a balanced diet you can ensure you're minimizing the fat-gaining effect. Focus on protein, whole food carbohydrates, healthy fats and vegetables. You should also make sure you're increasing your calories only a small amount at a time rather than all at once. This is where tracking will be key so you have specific numbers to hit rather than guessing. Yes, being in a calorie surplus takes a little effort and planning! It's not something you should do if you're not prepared to put in some work.
Keep up with your workouts and take progress photos so you can see changes over time. After a period of a few months, you can happily return to a higher-than-before maintenance and enjoy a bit more flexibility with your eating.